RMarkdown中ggplot图表渲染失败：无法获取PNG资源求助

• 环境：Windows 10（build 19045），R 4.3.1，使用tidyverse、ggplot2、bookdown等包编写RMarkdown文档
• 现象：此前表格、图表均可正常生成，近期插入ggplot图表时出现无法获取PNG资源、连接被拒绝的报错；文档输出至网络驱动器路径，但使用plotly::ggplotly转换图表可正常渲染
• 系统信息：

R version 4.3.1 (2023-06-16 ucrt)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19045)
...（剩余系统信息省略）

• RMarkdown代码：

---
title: "Test" 
author: |
  | Author: Author Name
  | Unit: Unit
  | Email: email address
  | Github Handle: handle
output:
  bookdown::html_document2:
      fig_caption: TRUE
      toc: true
      toc_depth: 3
      toc_float: true
      number_sections: true
knit: (function(inputFile, encoding) { 
      out_dir <- '../Reports/';
      rmarkdown::render(inputFile,
                        encoding=encoding, 
                        output_file=file.path(dirname(inputFile), out_dir, "GGPLOT_TEST.html")) })

link-citations: true
bibliographystyle: apalike
---

```{r setup, include = FALSE}
knitr::opts_chunk$set(
  #echo = FALSE,
  collapse = TRUE,
  comment = "#>",
  root.dir = "f:/Crash Analysis/Vehicle_Design_and_Injury_Severity/"
)
.libPaths("C:/Program Files/R/R-4.3.1/library")
#library(xfun)
library(bookdown)
library(tidyverse)  #For data wrangling
library(ggplot2)
library(scales)
library(plotly)

test chart below #

dat = mtcars
    #Make chart
    ##################################
    #Prepare data

    p = ggplot( dat, aes(x = mpg, y =wt)) +
        geom_line(aes(x = mpg, y =wt)) + 
        geom_point(aes(x = mpg, y =wt)) +
        theme(plot.title = element_text(hjust = 0.5)) +     
        theme(text = element_text(size = 16)) +
        guides(color=guide_legend(title="Body Type"))
    
    print(p)

- 报错信息：

[WARNING] Could not fetch resource http://wpdotfill09/R_VMP3_USERS/tdb069/Crash%20Analysis/Vehicle_Design_and_Injury_Severity/Reports/GGPLOT_TEST_files/figure-html/ggplot-1.png: HttpExceptionRequest Request {
host = "wpdotfill09"
port = 80
secure = False
requestHeaders = []
path = "/R_VMP3_USERS/tdb069/Crash%20Analysis/Vehicle_Design_and_Injury_Severity/Reports/GGPLOT_TEST_files/figure-html/ggplot-1.png"
queryString = ""
method = "GET"
proxy = Nothing
rawBody = False
redirectCount = 10
responseTimeout = ResponseTimeoutDefault
requestVersion = HTTP/1.1
proxySecureMode = ProxySecureWithConnect
}
(ConnectionFailure Network.Socket.connect: <socket: 792>: failed (Connection refused (WSAECONNREFUSED)))

Output created: ../Reports//GGPLOT_TEST.html

# 问题原因分析
核心问题是**knitr将网络驱动器上的图片路径错误解析为HTTP URL**：
- 输出到网络驱动器时，knitr把共享路径`\\wpdotfill09\...`转换成了`http://wpdotfill09/...`的HTTP请求格式
- 目标主机`wpdotfill09`未开启HTTP服务（80端口），导致尝试通过HTTP访问图片资源时被拒绝
- `plotly::ggplotly`生成的是内嵌在HTML中的交互式JS元素，不需要读取外部PNG文件，因此不受影响

# 解决办法
### 方案1：强制使用本地文件路径格式
在setup代码块中添加`base.url`参数，指定图片资源的本地网络路径（`file:///`开头）：
```r
```{r setup, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  root.dir = "f:/Crash Analysis/Vehicle_Design_and_Injury_Severity/",
  # 强制使用本地文件路径访问图片
  base.url = "file:///F:/Crash Analysis/Vehicle_Design_and_Injury_Severity/Reports/"
)
.libPaths("C:/Program Files/R/R-4.3.1/library")
library(bookdown)
library(tidyverse)
library(ggplot2)
library(scales)
library(plotly)

### 方案2：先输出到本地目录，再移动至网络驱动器
修改`knit`函数，先将文档输出到本地临时目录，再复制到网络驱动器：
```yaml
knit: (function(inputFile, encoding) { 
      # 本地临时输出目录
      local_out_dir <- tempdir()
      # 渲染到本地
      local_output <- rmarkdown::render(inputFile,
                        encoding=encoding, 
                        output_file=file.path(local_out_dir, "GGPLOT_TEST.html"))
      # 复制到网络驱动器
      file.copy(local_output, file.path(dirname(inputFile), '../Reports/', "GGPLOT_TEST.html"), overwrite = TRUE)
      # 复制图片文件夹
      file.copy(file.path(local_out_dir, "GGPLOT_TEST_files"), 
                file.path(dirname(inputFile), '../Reports/', "GGPLOT_TEST_files"),
                recursive = TRUE, overwrite = TRUE)
      })

方案3：禁用knitr的URL自动转换 #

在setup代码块中设置fig.path为相对路径，并关闭self_contained（避免尝试内嵌图片时出错）：

```{r setup, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  root.dir = "f:/Crash Analysis/Vehicle_Design_and_Injury_Severity/",
  # 设置图片相对路径
  fig.path = "GGPLOT_TEST_files/figure-html/",
  # 不生成单文件HTML，保留外部资源
  self_contained = FALSE
)
.libPaths("C:/Program Files/R/R-4.3.1/library")
library(bookdown)
library(tidyverse)
library(ggplot2)
library(scales)
library(plotly)

### 方案4：使用绝对路径直接指定图片存储位置
在setup代码块中设置`fig.path`为网络驱动器的绝对路径：
```r
```{r setup, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  root.dir = "f:/Crash Analysis/Vehicle_Design_and_Injury_Severity/",
  # 直接指定图片存储到网络驱动器的绝对路径
  fig.path = "F:/Crash Analysis/Vehicle_Design_and_Injury_Severity/Reports/GGPLOT_TEST_files/figure-html/"
)
.libPaths("C:/Program Files/R/R-4.3.1/library")
library(bookdown)
library(tidyverse)
library(ggplot2)
library(scales)
library(plotly)

---

内容的提问来源于stack exchange，提问作者Josh R.